vp6dec: ensure we don't try to use a buffer with negative size this should never happen in valid vp6 bitstream fix issue684
vp6dec: ensure we don't try to use a buffer with negative size
this should never happen in valid vp6 bitstream
fix issue684


git-svn-id: a4d7c1866f8397a4106e0b57fc4fbf792bbdaaaf@15610 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg